What Lies Beneath: The Top 3 Intrusive Site Investigation Methods
Intrusive site investigations provide a firsthand look at what's beneath the surface, and using these methods, one can gather information unobtainable with non-invasive methods.
Site investigation encompasses a variety of investigative methods that identify risks to a project. These methods can be non-intrusive or intrusive in nature. General objectives of site investigation are: to study past developments that could affect new projects to be developed at the site, identify environmental and ecological implications, and determine the need and methods for intrusive site investigation.
Site investigation is broadly classified into four stages, namely: reconnaissance, data and map study (desk study), in-depth or intrusive investigation and laboratory testing. Here we will take a look at a few intrusive site investigation methods. (To learn more about site investigation, see Understanding the 4 Stages of Site Investigation.)
A trial pit is a shallow intrusive investigation method. Other shallow methods include digging observation pits and trenches (to determine the precise location and depth of the identified utility), and inspection pits (to investigate the buried portions of superstructures and foundations).
Trial pits are excavated to sample and investigate superficial deposits up to a maximum depth of 4-5 meters, and in soils that can stand unsupported – shoring can be provided, but can prove to be expensive.
Trial pits can indicate lateral and vertical variation in ground condition that is not possible to detect by other methods. Samples can be collected for lab testing and index tests. The soil vane shear test (VST) can be carried out on chunks of cohesive soil brought out in the excavator bucket.
The trial pit record includes stability of excavated sides, orientation, position and surface elevation of the pit, representation of encountered strata including lateral variation, and samples of groundwater, if encountered.
Used extensively in site investigation, boreholes penetrate to deeper strata than trial pits while causing less disturbance. Boreholes can penetrate through any type of strata, and the most commonly used methods for drilling boreholes are percussion boring and rotary drilling.
Cable Percussion Boring
Cable percussion boring is the most convenient and common method of intrusive geotechnical investigation, and depending on ground conditions and access to the site, boreholes up to 60 m depth can be drilled. The rig consists of a winch and a derrick that can be folded down and towed to and from the site.
For cohesive soils, the borehole is drilled using a clay cutter, and for non-cohesive soils, a shell is used. A sampler is driven by percussion using weights to advance the borehole into the ground. The soil sample can be retrieved by bringing up the sampler.
The borehole sides are supported using casing as the borehole proceeds. For very hard ground that is difficult to penetrate or for obstructions such as boulders, a chiseling tool can be used. Different sample types can be retrieved using this method such as cohesive disturbed samples, granular disturbed samples, undisturbed samples, piston samples and split barrel sampler (used for the standard penetration test (SPT)).
Tests such as the SPT, borehole vane test and falling or rising head test can be carried out within the borehole. (For more on geotechnical investigations, check out The Geotechnical Investigation: Which Method Should We Use?)
Rotary drilling is a method of drilling a borehole by rotating a drill bit at its bottom. Though preferred for rock sampling, it can be used for soil sampling by using the correct drilling method. Drilling fluid is passed through the drill rod and is used to lubricate and cool the drill bit and also to transport the cuttings to the surface.
Rotary drilling can be open-hole drilling or core drilling. Open-hole drilling does not recover any usable samples for testing and is usually used to locate voids, holes or mine shafts. Core drilling is used to retrieve usable samples for geotechnical investigation. A rotating core barrel fitted with an annular bit is used.
The core barrel retains the core, which is brought to the surface and recovered.
The probing method uses a steel rod about 25 mm in diameter that is driven into the ground to probe or sound. The resistance to driving the rod, and the particles that stick to the rod when it is pulled out, indicate the properties of the soil.
This method provides a continuous ground profile and is more accurate compared to other methods. There are two types of probing:
Dynamic probing utilizes a rod with a cone of a slightly larger diameter than the rod, fitted at its base. Using a constant mass and force dropped from a standard distance on the anvil at the top of the rod, the rod is driven into the ground. The number of blows required to drive the cone every 100 mm is recorded.
This method helps determine the thickness and distribution of strata at different locations, since it is portable and easy to set up and operate. It, however, does not identify soil composition, nor can it retrieve soil samples.
Like dynamic probing, the static probing method involves penetrating the soil using a rod attached to a steel cone. However, instead of a weight being dropped on the anvil, the rod advancement is achieved using a static push. To achieve the high pressure to drive the rod, equipment with high mass is required.
The cone head is fitted with a sensor that records the driving resistance; this is possible because the rate and force of the drive is kept constant during the test. A friction jacket is fitted behind the cone and can be advanced independently of the cone. An electronic piezometer is also attached to the driving assembly.
This complete system is now capable of recording soil type variation, soil strength and groundwater pressure. Additional tools allow soil samples to be recovered for inspection and testing.
Any type of geotechnical investigation is carried out to understand the properties of the soil in order to help design better subsurface infrastructure. In order to completely analyze the soil characteristics and strength, a combination of the above methods should be used. Site investigation is critical for every project and due attention and time should be allotted for it.
Using geotechnical reports of previous projects at the site to save time can prove to be a costly mistake. Every project is unique and should be treated as such.